import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.Comparator;
import java.util.PriorityQueue;
import java.util.Scanner;

/**
 * 
 */

/**
 * @author zhongfang
 *
 */
public class Main{
	public static class Score{
		public Score(int i, int j) {
			this.originalScore=i;
			this.testTime=j;
		}
		int originalScore;
		int testTime;
	}
	//用贪心的思想求解，优先复习性价比高的学科直到该门课满分
	/**
	 * @param args
	 */
	public static void main(String[] args) {
		Scanner scanner=new Scanner(System.in);

		while (scanner.hasNextInt()) {
		int n=scanner.nextInt();
		int r=scanner.nextInt();
		int avg=scanner.nextInt();
		long goal=((long)n)*avg;//剩下要完成的分数目标

		long result=0;//需要复习的总时间
		PriorityQueue<Score> queue=new PriorityQueue<>(n, new Comparator<Score>() {
			@Override
			public int compare(Score o1, Score o2) {
				if(o1.testTime==o2.testTime)
					return 0;
				else if (o1.testTime<o2.testTime) {
					return -1;
				}else {
					return 1;
				}
			}
		});

		for (int i = 0; i < n; i++) {
			int a=scanner.nextInt();
			int b=scanner.nextInt();
			Score score=new Score(a, b);
			queue.add(score);
			goal-=a;
		}
		
		if(goal<=0)//平时分就够达标，无需复习
			System.out.println(0);
		while (goal>0) {
			Score curScore=queue.remove();
			long thisAchive=Math.min((r-curScore.originalScore), goal);//这门课的目标分数，是剩下达到该课程总分或者达到总目标
			long time=(long)thisAchive*curScore.testTime;//对这一门复习的时间
			result+=time;
			goal-=thisAchive;
		}
		System.out.println(result);
		}
	}

}
